The issue here is baptism into a church, and that is delayed. If we were to assume that Ammon had the priesthood and the right and authority to baptize, it is hard to imagine a reason why he would deny baptism to a people who so clearly desired it, and were prepared for it. Peterson suggests that it is because Ammon is a warrior (Peterson, Daniel C. "Priesthood in Mosiah." The Book of Mormon: Mosiah, Salvation Only Through Christ. Religious Studies Center, BYU, 1991, pp. 201). The more probable answer would appear to be Ammon's unfamiliarity with the specifics of what the Limhites desired. Note that Peterson agrees that the institution of a "church" would be a new innovation with Alma (Peterson, p. 200).